Method and means for the electro-deposition of chromium



Patented Mar. 4, 1952 METHOD AND MEANS FOR THE ELECTRO- DEPOSITION F CHROMIUM Floricel A. Rojas, N ew York, N. Y.

No Drawing. Application December 22, 1945, Serial No. 637,122-

20 Claims.

The present invention relates to a new and improved method for the electro-deposition of chromium and to a new and improved plating bath 'and catalyst for use in carrying out such method.

This application is a continuation-in-part of my copending application Serial No. 341,970, filed June 22, 1940, which was substituted for my earlier application Serial No. 186,655, filed January 24, 1938, both of which applications have been abandoned.

It is an object of my invention to provide a new and improved method for the electrodeposition of chromium directly on the article to be coated without the necessity for providing the article with a primary or undercoat of copper, nickel or the like, as is widely practiced today, except where porous surfaces are present.

Another object of my invention is to provide a new and improved catalyst for use in a chromium electro-plating bath.

A further object of my invention is to provide a new and improved bath for the'electro-plating of chromium and having the advantages set out below.

It is also an object of the invention to provide a chromium plating bath which is characterized by the capacity for continuously producing superior chromium platings without the addition thereto of strong inorganic acid like sulfuric, which. operates highly satisfactorily also in the absence of organic acid, and which is self-regulating with respect to the amount of strong acid ion, like sulfate ion, in solution, the same being automatically controlled within limits mined by the solubility in the bath of a relatively insoluble or sparingly soluble metal salt, such as a sulfate, chloride or fluoride.

Other and further objects of my invention are to provide a bath and'method for the electrodeposition of chromium in which a very wide range of current densities may be employed; in which continuous plating may be carried out commercially with the plating bath operated at various temperatures from room temperature to the boiling point; in which there is substantially no local galvanic action or high local polarization at the cathode, thereby improving throwing power; in which the fuming and spray losses are very materially reduced over present practices, and a bath which can be adapted readily to give either very hard or relatively soft; nonporous. coatings of chromium, in both cases of fine, grain structure, great adherence, and free from occluded basic oxides.

deter- Further objects of my invention are to provide a bath and method for the electro-deposition of chromium which can be operated without any need for making pH adjustment of the bath, and without the necessity for adding free sulphuric acid to the bath; which bath is continuously operable over long periods of time without other attention than the replenishment of chromic acid (and of any catalyst that may be lost to the bath) to maintain a selected bath density; which prevents corrosion of the article to be electro-plated; which provides an improved current efiiciency and increased throwing power.

Still other and further objects of my invention are to provide a bath and method for the electro-depcsition of chromium in which the chromium ion is protected from contamination while depositing metallic chromium on the article acting as the cathode in the bath; in which the amount of trivalent chromium at the cathode is controlled by the secondary ionization of the catalyst; in which thicker deposits of chromium are obtained in the same time than are now secured commercially, or the same depth of deposit is obtained in less plating time; and in which the character of the plate can be controlled and the nature of the cathode metal taken into account by suitable selection of the catalyst.

Other and further objects and advantages of my invention will become apparent from the description hereinafter and the features of novelty will be set out in the appended claims.

I have found that electro-deposits of chromium can be produced on cathodes of difierent kinds having conducting surfaces, which deposits are dense, non-porous, and of extraordinary adherence, if the bath contains a metal chromate or dichromate which is soluble in chromic acid solution, and preferably also an excess of a relatively insoluble salt of still another metal, and that superior plating results are attained if such compounds are prepared by metathesis. By suitable selection of the soluble chromate or dichromate, or of mixtures of such compounds, chromium coatings which excel for purposes of decoration and/or corrosion resistance and/or erosion resistance can be obtained. 7

The electro-plating bath of the present invention is characterized by the presence, in addition to the chromium, of at least two other metals. One of these metals is in combination in the form of a soluble dichromate (or a solution of its chromate in the chromic acid solution). Examples of such, soluble dichromates are those of zinc, nickel, aluminum, cadmium, trivalent chromium (as distinguished. from the hexavalent chromium of the chromic acid or chromic oxide used in making up the plating bath) magnesium, tin, trivalent molybdenum, lithium, sodium and potassium, the dichromate of the alkali metals being generally less desirable than those of the less electro-positive metals. The second metal is one which will form a relatively insoluble salt but will supply what appears to be a catalytically acting ion, or a component of a catalytically acting more complex ion, to aid in the deposition of the chromium in a satisfactory form. Examples of these metals are barium, strontium and calcium and the relatively insoluble salt of these metals which is preferred, being the sulphate, barium sulphate generally yielding bestresults. However, the second additional metal need not be one of the three just mentioned, nor need the salt be the sulphate. Thus catalytic compositions wherein the relatively insoluble compound is silver chloride or strontium fluoride have been found by me to give fairly satisfactory results, although generally not as good as those obtained with relatively insoluble metal sulphates.

The plating bath of the present invention will thus contain, in addition to the excess chromic acid (chromic oxide in aqueous solution) and metal dichromate in solution, a dissolved quantity K of a relatively insoluble compound of a third metal corresponding to the solubility of such compound in the bath. Whether the third compounds are present in solution as such individual compounds cannot definitely be determined; but it seems likely that some complex radical is formed from which the chromium is electrolytically deposited onto the cathode, and this seems to be indicated by the fine structure of the chro-' mium deposit. It may be that because of the formation of a complex ion, the relatively insoluble compound has a higher degree of solubility than it would have in pure water. The relatively insoluble compound provides, as I have found, a catalytically acting ion whose concentration in the bath solution is determined by the limited solubility of such compound and which in association with the soluble dichromate and the chromic acid promotes the deposition of an improved chromium plate.

An important phase of my invention accordingly resides in the preparation of the catalyst composition from which the bath is prepared. While theoretically it would seem that the mixture comprising my catalyst might be made up simply by mixing the ultimate components of the catalyst composition as they may be obtainable in commerce, I have found that consistently reliable results are best obtained by preparing the catalyst by double decomposition in such manner that a soluble metal dichromate or chromate (which may form the dichromate in the chromic acid solution in which the catalyst is formed) is formed together with the relatively insoluble metal compound above referred to.

While I do not wish to be understood as being committed to any particular theory, my improved catalyst may be regarded as a complex composed of three metals and suitable acid radicals, the

compounds appearing to be united or associated molecularly to form a substantially neutral ionizable salt complex, which I introduce into the electro-plating bath. The catalyst composition of the present invention is preferably prepared by reacting together two solutions. A and B.

prepared with compounds which may have undergone double decomposition therein, to produce a reaction product C containing the products of double decomposition, one part of which is soluble (the dichromate), and the other part relatively insoluble (the sulfate, chloride, fluoride,

etc.).

Reaction product A may be produced, in one form of the invention, by treating a mixture of a soluble metal salt and a base, such as the oxide or carbonate, of the same metal with sufiicient chromic acid and water to dissolve the same and permit their complete mixing and interaction if necessary, at high or boiling temperatures.

Reaction product B is made by treating, for example, an alkaline earth metal salt or oxide, and a base, such as the oxide or carbonate or hydroxide of the same metal whose base is used in product A, in sufficient chromic acid and water to dissolve all the materials and permit their complete interaction, the resulting salts being maintained in solution if necessary with the aid of heat or of suitable intermediary agents, like ammonia in the case of silver.

Reaction product C is then made by mixing together reaction product A and reaction product B, if necessary boiling the resultant product for a time sufiicient to have all of the ingredients react to complete equilibrium. Reaction product C then may be aged to insure that complete chemical equilibrium is reached before use as a catalyst in the plating bath.

Graphically, reaction products A and B can be represented by the following formulae:

(A) RZ+R oxide+CrO3+Hz0 (B) XY-l-R oxide+CrO3+H2O in which R is a metal which can form a salt with chromic acid and whose dichromate is soluble in water, such as zinc, aluminum, nickel etc.; Z is a suitable acid radical, such as sulphate, chloride, fluoride, etc.; X is a metal such that it will form an insoluble salt with the radical Z of'reaction product A, such as the alkaline earth metals barium, strontium, and calcium; and Y is an anion which is readily decomposed and removes itself from further action in the reaction product 0, such as the carbonate radical (003) of the selected alkaline earth metals. The oxide of the metal may be replaced by the hydroxide or by the carbonate in some instances to facilitate solution in the chromic acid.

As set out hereinbelow, the catalyst or composition of the present invention may or may not have incorporated therein all of the chromic anhydride required for making up the plating bath; and if only enough chromic anhydride is employed in making up products A and B to form the zinc or equivalent dichromate, the additional chromic anhydride is added when the plating bath is made up. It is preferred that even where the product C does not contain all of the excess chromic anhydride necessary to make up the complete plating bath, a suflicient excess be used in making up products A and B to facilitate the reaction. The amount of excess chromic anhydride in the products A and B and in the final reaction product C can thus be variable up to the full amount necessary to make up the complete bath.

In preparing the plating bath utilizing the catalyst of the present invention, the bath is adapted for the different kinds of chromium coatings while preserving all of the foregoing advantages over long periods of use by suitable selection of the diiierent metal compounds. The catalyst composition, in whatever amounts are required for the particular character of coating, is added to chromic acid in solution (if such composition does not already contain sufficient chromic acid to start, the bath), the mixture is agitated, and the bath adjusted to the proper specific gravity (usually from to Be.) by the addition of water., It is then desirable to heat the bath to a temperature of approximately 180. F. or higher to hasten the attainment of chemical equilibrium. The bath is then ready for continuous use for the direct electro-deposition of chromium upon objects acting as the cathode in the plating bath at any temperature suitable for" electroplating as hereinafter set forth. The anodemay be lead-antimony alloy (94%, and 6% antimony) or any other metal or alloy insoluble in the bath and below chromium in the electroinotive series of metals.

Considering the catalyst itself as comprised of the soluble metal dichromate (or metal chromate withsuflicient chromic oxide to convert it into the dichromate), the precipitated salt, and any soluble molecular complexes that may be formed (as distinguished from the mixture containing also excess chromic acid, which I designate as the catalyst composition), the ratio between the amount of such catalyst and the quantity of chromic acid in the bath may vary over very wide limits; and correspondingly, the ratio of the metal of the diehromate of the catalyst to the total free chromic acid in the bath may vary over a wide range, such relationship not being critical. Thus relatively small proportions of the catalyst may be used in making up the bath, or only just suificient chromic acid may be present to insure proper and adequate deposition of chromium on the cathode. As the catalyst composition is not consumed during the electrolysing of the bath, as small a quantity can be used as will be sufiicient to insure proper plating in any particular bath. In general, it is preferred that the bath contain about 2 to 10 parts of the metal of the soluble dichromate (zinc, nickel, aluminum,

Example 1 R .eaction product A is prepared by mixing and reacting the following:

287.5 gr. crystalline zincsulphate (ZnSOrJZHzO) 81 4 gr. zinc oxide (ZnO) 400 gr. chromic anhydride (crop 250- gr. Water Reaction product B is prepared from the'following;

197.4 gr. barium ca bonate (BaCOs) 81.4 gr. zinc oxide (ZnO) 600 gr. chromic anhydride (CI'Os) 360 gr. water It will be noted-that the quantities" of 'zinc sulphate and barium carbonate are equivalent, i. e., that they represent combining proportions, so that all of the sulphate ion is bound by barium, it being preferred not to use an excess of zinc sulphate over the formed barium dichromate, or vice versa.

The ingredients of reaction product A are added to the water, and the water heated to prefera-bly the boiling point, in a suitable container cess of chromic anhydride, will contain barium dichromate. In both mixtures A and B the reactions are not complete until the solutions have become clear, true solutions and contain no unclissolved or suspended material. In both of these products A and B, the chromic anhydride is in excess of coinbiningproportions with reference to the 2110, including that contained in the zinc sulphate.

While hot. the solutions of the reaction products A and B are then mixed together in a suitable vessel With proper agitation, and preferably boiled for filteen minutes or a time sufiicient to have all of the ingredients completely react together. This operation will cause chemical reaction to occur, precipitating barium sulphate, while the remainder continues in solution. The soluble and insoluble salts constitute the catalyst. Thus there is produced a complex salt mixture containing the three metals, with the with the above product C, I add to a selectedamount of product C, whose zinc and free chro niic anhydride content can be easily computed, a sufficient amount of chromic anhydride to give.

the desired ratio of zinc to the anhydride. Enough water is added to give a bath density of, say 20 Be. The bath shouldbe heated to hasten reachingchemical equilibrium as heretofore described. The bath is now ready for use.

. The proportion of catalyst is selected accordof about -140 F.

Under different conditions of bath operation, different results will be obtained, and in fact, in certain cases, results may even bereversed. It it therefore impossible to specify accurately the optimum proportions for all conditions of operation, such optimum conditions being best determined by simple experiment taking into account the desired temperature of operation, the current density, the distance between the electrodes, the cathode contour, the size ofthe anode, the type of coating desired, the nature of the surface on which the plating is to be deposited, etc. A satisfactory proportion for producing light coatings under average conditions is about parts of combined zinc to 100. parts of chromic anhydride.

The above plating bath will operate at all temperatures from room temperature to close to the boiling point of the solution and at a given temperature will function with great current efficiency and at a high current density, with wide ranges of current density being permissible. For example, at 70 F., this bath will plate continuously and successfully at a cathode current density of from 12 to '70 amperes per square foot; at 100 F., at from to 200 amperes per square foot; and at 140 F. at from 140 to 1000 amperes per square foot. For normal plating I prefer to operate at 140 F. with a current density of 700 to 1000 amperes per square foot. In the .ranges given, continuous, white deposits of fine grain, non-porous, non-cracking and non-peeling metallic chromium will be obtained.

Example 2 If desired, the catalyst composition may be prepared with sufficient chromic anhydride so that the electroplating bath may be formed directly therefrom without the further addition of chromic acid. In such a case, for example, reaction product A may be made according to the following formula:

300 gr. crystalline zinc sulphate 218 gr. zinc oxide 1500 gr. chromic anhydride 5000 gr. water Reaction product B will be made as follows:

200 gr. barium carbonate 218 gr. zinc oxide 3500 gr. chromic anhydride 5000 gr. water Example 3 Reactant product A is prepared from the following: 287.5 gr. crystalline zinc sulphate (Zl'lSO4.7H2O) 81.4 gr. zinc oxide 400.0 gr. chromic anhydride 250.0 gr. water Reactant B is made up from the following:

197.4 gr. barium carbonate 81.4 gr. zinc oxide 600.0 gr. chromic anhydride 360.0 gr. water Solutions A and B are mixed, yielding product C having the following composition:

844.2 gr. zinc dichromate 233.4 gr. barium sulphate 400.0 gr. chromic anhydride (free) 148.0 gr. water (10 to 12% of solid matter of product C) 1625.6 gr. weight of catalyst composition chromic anhydride. In preparing a final plating bath which is to contain a selected quantity of combined zinc for each grams of free chromic anhydride, the quantity of free anhydride in the catalyst composition should be taken into account, as indicated above.

Example 4 Reactant A is made up from the following:

282.84 gr. crystalline nickel sulphate (NiSO4.6H2O) 50.0 gr. chromic anhydride 300.0 gr. water Reactant B is prepared from the following:

315.5 gr. barium hydroxide (emonnsmo) 350.0 gr. chromic anhydride 500.0 gr. water Product C will then be composed as follows:

779.1 gr. weight of catalyst The total amount of combined nickel in product C is 58.7 grams. In making up a plating bath from this product C the quantity of free chromic acid in such product will, of course, be taken into account. This product when mixed with additional chromic anhydride to an extent such that about 5 to 6 grams of combined nickel are present for every 100 grams of chromic anhydride and diluted to a density of about 20 B. is especially adapted for producing decorative chromium of unusual hardness and brilliance, especially on iron or steel articles.

Example 5 Reactant A is made up from the following:

666.4 gr. aluminum sulphate (A12SO4)3.18H2O) 50.0 gr. chromic anhydride 350.0 gr. water Reactant B is prepared from the following:

946.5 gr. barium hydroxide (Ba(OH) 2.8H2O) 600.0 gr. chromic anhydride 350.0 gr. water 9 Solutions A and B are combined and yield prodnot C having the following composition:

1602.2 gr. weight of catalyst composition Product C contains 2 gram atomic weights. of aluminum, and the small amount of free chromic anhydride that it contains should be taken into account in making up a plating bath therefrom. This composition containing aluminum dichromate isespecially suited for plating on metals which are displaceable by aluminum, such as iron, zinc, nickel, copper, etc. The plating bath has high throwing power and .high current Qfli fiency'. This bath has the widest general applicability although it does not have as good throwing power as the zinc dichromate baths.

Example 6 Rea'ctant A is a simplesolution made up qf the iel owing: 600.0 gr. chromic anhydride 300.0 gr. water .Reactan B i p e a ed "b rea ti g he f i ing substan es; 946.5 gr. barium hydroxide (Ba (OI-I) 2.8H 2O) 666- r. aluminum ulph te A1 Q4 -,l HZQ 30.0...0 gr. water Upon mixing of these two solutions, product C is obtained which has the following composition:

702.0 .gr. aluminum dichromate 700.2 gr. barium sulphate 1 5.0.0 j r. water to 12% of solid matter of product .0)

gr. weight of catalyst The preparation of this catalyst diiTers from that of the preceding examples, in the first place, in that solution A contains only one combined metal. Also, the barium is not in the form of barium di-chromate, but is used in the form of barium hydroxide in order to convert the aluminum sulphate into aluminum hydroxide which can more readily react with the chromic anhydride to aluminum dichromate than would be the case with the aluminum oxides generally available on the market. Itwill, however, be ob served that the barium sulphate is formed, so to speak, in situ by double decomposition, and that the aluminum dichromate is similarly formed.

The product C of this example is all catalyst, that is, there is no excess chromic anhydride. The-total weight of combined aluminum in the 155-22 grams of product C is 54 grams or 2gram atomic weights.

The proportion of combined aluminum in the catalyst being known, it is a simplematter to determine how much catalyst is needed for every -100 grams of chromium anhydride to give the desired ratio between the combined aluminum and the anhydride. This bath 'has generally the same advantages as those set forth in connection with Example 5.

It may be noted that whereas in the previous examples all of the solutions A and B were quite clear-after the reactions involvedin their-formation were completed (assuming, of course, that pure chemicals were employed) in solution "Bof 10 Example 6 aluminum is in the form of the waterinsoluble hydroxide, while at the same time barium is present as the sulphate, so that reaction product 13 will not be clear.

It may be observed that whereas in Examples 1, 3, 4, 5 and 6 the ratio of the quantity of the catalyst or catalyst composition to that of the added chromic anhydride is a finite quantity, in Example 2, Where no free chromic anhydride need be added to make up the chromium composition, the ratio is infinite.

Example 7 Fairly satisfactory results can be obtained also by the metathetical production of a soluble dichromate whereby at the same time there is formed a more or less insoluble salt of an acid other than sulphuric. Thus a product C containing zinc dichromate with or without excess chromic anhydride and containing also a precipitate of silver chloride can be obtained by combining two solutions, one containing zinc chloride and the other silver dichromate, followed by boiling to insure complete reaction. Solution of the silver dichromate may, if desired, be promoted with the aid of an intermediary like ammonia, which is ultimately driven off. As in the case of the examples given above, the quantities of the substances entering into the double decomposition are substantially equivalent, so that practically no excess zinc chloride is contained in prod- .uct C. l

Example 8 In similar fashion a product C containing zinc dichromate (or other soluble dichromate) and Example 9 Eq iva ent pr portio s of al ium .dichroma can be substituted for the barium dichrojmate used inthe preceding examples, product C then remaining zinc ldi hromate and the sparin ly soluble ,calicum sulphate, with or without excess .Qhmmic anhydride. terial yield acceptable plating, but the results obtainedwith the catalysts produced .by metathesis between '='bariu m dichromate and the sulphate .of va-metai whose dichrcmate is soluble, are generally superior.

The temperature and the current density range used will depend upon the ultimate use of the object being plated, for example, in the flash platingcf ,drills and similar small tools a iew-se cands of electroplating close to the boiling point'of the .bath and with a current density of 1500 .amperes per squ feet w l iv a co t nuous, .thin coatof adherent, brilliant chromiumwhich will-,bevery'hard andsmQoth. For coating articl s wit chro ium t re t rr ion an erosion. platin can b d n ent e sat siacto r ily-at ,room temperatures with a current density .o -fioampe es perscr ar ioot- As1-wi b ap rec e f o the f oin Q9 of thead an a es o he esenti tio t a the bath b suc essfu l Q e t th u iih re enceo added ziree lphur acid e e 5 ubl meta su ph t The hat ll h e er Baths made with this macontain sulphate ions, and I have obtained best results with that concentration of sulphate ions which corresponds to the solubility of barium sulphate in the bath. However, a concentration of such metal-bound ions corresponding to a sulphuric acid acidity of as much as 0.01% to 0.2%, based upon the chromic acid content, will not interfere with the satisfactory operation of the bath and can be secured, as already indicated, by the use of somewhat more soluble metal sulphates than barium sulphate, for example, strontium and calcium sulphates, or mixtures of these sulphates, produced by double decomposition in situ. At higher concentrations of sulphate ion, poorer coatings will generally be obtained. As already mentioned, the absence of added sulphuric acid eliminates local galvanic action and greatly improves the behavior of the bath. The absence of such galvanic action, and the fact that my baths are characterized by high degree of local polarization, especially where zinc dichromate is present in the bath, endows the bath with greater throwing power than is possessed by the baths hitherto used for the electrodeposition of chromium, this throwing power being so effective that recessed articles and articles of complex contour can be more easily plated with more nearly uniform depositions of chromium without the use of specially designed auxiliary anodes.

As set out hereinabove, the catalyst of the present invention may or may not have incorporated therein all of the chromic anhydride for making up the plating bath; and if only enough chromic anhydride is employed in making up products A and B to form the zinc or equivalent dichromate, the additional chromic anhydride is added when the plating bath is made up. It is preferred that even where the product C does not contain all of the excess chromic anhydride necessary to make up the complete plating bath, a sufficient excess be used in making up products A and B to facilitate the reaction. The amount of excess chromic anhydride in the products A and B and in the final reaction product C can thus be variable up to the full amount necessary to make up the complete bath.

As chromium metal is plated out of the bath, the bath is replenished by the addition of chromic acid (CrOs). The catalyst needs replacement only of that amount mechanically removed from the bath by adhering to the plated articles withdrawn from the bath, and needs replenishment at only long intervals of time.

The plating bath of the present invention operates to deposit chromium directly on the cathode, that is, without an intermediate deposit of another metal on the cathode. If the theory that a metal can be plated only out of a cation containing the same is correct, it is diificult to explain the mechanism of the plating accomplished by the present invention. It may be that somehow a small quantity of chromous oxide is present in the bath, being either present as an impurity in the chromic anhydride, or having been produced by reduction by organic impurities, such as exist in the air. Whatever the origin of the chromous ion is, it appears to form (chromous) chromium bichromate, and it is apparently from the cation of this salt or from its cationic combinations with the other metals in the bath that the chromium is deposited on to the cathode. Apparently, in the case of a bath containing a dichromate of zinc or other metal which is higher than chromium in the displacement series, the zinc or equivalent metal is deposited first but immediately displaces the chromous ion in solution and is itself in turn ionized, which phenomenon may be designated as secondary ionization. In the course of these phenomena the barium sulphate plays a role in promoting the chromium deposition and in determining the character of the deposit. Just what this role is, cannot be definitely determined. It appears, however, to be of a catalytic nature because I have found that when a bath prepared, for instance, according to Example 1 hereinabove, but from which the barium sulphate precipitate has been removed, is operated intermittently over a period of several months, it loses its efficiency; but if a quantity of water which has been stirred with the original precipitate is decanted from the precipitate and poured into the plating bath, the latter is revivified. Barium being higher than chromium in the displacement-series can possibly be initially deposited on the cathode, thereafter acting to displace dissolved chromous ion and being itself re-ionized, so that a kind of tertiary ionization takes place. If the-barium in fact deposits, it probably does so before the zinc. However, these explanations are purely speculative and are not to be construed as affecting the validity of the invention, the workability of the latter having been definitely established in practice.

One of the characteristics of my improved chromium plating bath is the reduced generation of hydrogen at the cathode. It appears that the hydrogen formed at the cathode is converted into water by reaction in the atomic condition with the chromic anhydride, reducing the chromium to the trivalent condition. This reduced generation of hydrogen is highly desirable because gaseous hydrogen is a very poor conductor of electrical current, so that the elimination of the accumulation thereof around the cathode, commonly known as hydrogen polarization, greatly contributes to the increased current efficiency of my bath, insures a continuous and uninterrupted plating operation, contributes to permitting the use of higher current density, allowsready access of the chromium to recessed parts of the object being plated, and, as already mentioned, controls, i. e., 'keeps at a minimum the amount of trivalent chromium that is formed, so thatthe accumulation of trivalent chromium is prevented.

These recessed parts normally have the greatest accumulation of gaseous hydrogen which blocks the chromium plating therein. While permitting local polarization at parts of an irregular cathode nearest the anode, such as the relief parts of the cathode, the film of hydrogen at the recessed parts is reduced or eliminated by the formation of water in the use of my bath.

I have already indicated in the foregoing that the proportion of zinc to free chromic anhydride can vary rather widely. A practical range of variation is represented by the following limits:

In reaction product A, the zinc sulphate may be varied between 41 and 287.5 grams, while the zinc oxide maybe varied from 12 to 81.4 grams for every 400 grams of chromic anhydride, reference being had particularly to Example 1. In reaction product 13, the barium carbonate may be varied from 28 to 197.4 grams and the zinc oxide from 0 to 81.4 grams for every 600 grams of chromic anhydride. However, in each case the quantity of barium carbonate and zinc or equivalent sulphate should be maintained equivalent, so that all sulphate radical is bound to barium. The quantity of water used in each case is preferably that just sufiicient to dissolve the ingredi- 13 en'ts and permit complete reaction at least at or near the boiling point. In addition to the-metals above mentioned whose bichromates are employed in my improved baths, there may be mentioned copper, which likewise forms a soluble bichromate. However, baths prepared with copper bichromate do not operate as well aswith, for example, zinc, nickel or aluminum. It will be borne in mind that in making substitutions of one metal for another in any of the above formulae it is necessary that the proportions be changed in accordance with their chemical equivalent weights.

While reference has been made hereinabove to the direct electro-deposition of chromium on the article acting as the cathode, without the provision of an undercoating of some metal, it will be understood that if in some cases an undercoating .isfound desirable, the above process and bath will function to deposit chromium on the thus coated article in the same manner as described above.

As already mentioned, the bath of greatest general application is one containing aluminum dichromate. Where greater throwing power is required, the zinc dichromate baths are preferred; while for brilliance and flash plating nickel d-ichromate baths have generally proved superior, provided that the contour of the cathode is not too irregular. Where relatively heavy platings are. requiredfor corrosion, resistance, the aluminum bichromate baths generally give best results with the zinc dichromate baths being better where high throwing power is also required.

Where particularly fine grain structure in the coating is required, it is of advantage to employ s:

baths containing a mixture of dichromates, for .example, aluminum and zinc dichromates, nickel and. aluminum dichromates, zinc and nickel dichromates, and zinc, aluminum and nickel dichromates. Other soluble dichromates may also be incorporated in these mixedcatalysts. The proportions of these mixtures are not critical, but the use of mixtures containing chemically equivalent proportions of the diflerent dichromates is preferred. However, in these mixtures the same roles'with regard to the presence of the sulphate ion only in the amount equivalent to a metal forming therewith a relatively insoluble sulphate is to be observed. It is preferred that all of the metals whose dichromates are contained in the catalyst be higher than the metal (cathode) being plated on in the displacement series.v

In the foregoing the examples of the relatively insoluble salt produced during the formation of the catalyst have been the sulphate, chloride and .fiuoride of certain metals. As the methathesis itself seems to impart, at least in large measure, the advantageous properties of my new catalyst,

the formation of other insoluble compounds by dures analogous to those described above, that is, .byreaction between, for example, the bichromate of a metal. whose phosphate is insoluble with the phosphate of a metal which forms a soluble bi- .chromate.

- I=claim.: '1. A composition for use in a bath for the elec- -trodeposition of chromium comprising the interacted mixtur'eof (1") the aqueous reaction product of-400 grams of .chromic anhydride, 41. to 287.5

grams of crystalline zinc sulphate, 12 to 81.4 grams of zinc oxide, and sufficientwater to .dis-

solve the ingredients and permit their reaction, and (2) of 600 grams of chromic anhydride, .28 to 197.4 grams of barium carbonate, up to about 81.4 grams of zinc oxide, and sufficient water to suspend the ingredients and permit their reaction, the proportions of sulphate radical and barium radical being substantially equivalent.

2. The method of preparing a composition for use in a bath for the electrodeposition of chromium which consists in mixing together the sulphate and oxide of one of the metals selected from the group consisting of zinc, nickel, and aluminum, with chromic anhydride and water, separately mixing together the carbonate of one of the metals selected from the group consisting of barium, strontium and calcium, with the selected oxide of zinc, nickel and aluminum, chromic anhydride and water, and reacting together the products of the two mixtures, the sulphate and carbonate being employed in substantially combining proportions, the quantity of chromic anhydride being at least sufficient toreact completely with all of the oxide and carbonate.

8. The method of preparing a composition for use in a bath for the electrodeposition of chromium which consists in mixing together at boil.- ing temperature 250 grams of water, 400 grams if chromic anhydride, 287.5 grams of crystalline zinc sulphate, and 81.4 grams of zinc oxide-separately mixing together at boiling temperature 360 grains of water, 600 grams of chromic anhydride, 197.4 grams .of barium carbonate and 81.4 grams of zinc oxide, and boiling together the product of the. two mixtures until complete reaction has occurred.

4. An aqueous bath for the electrodeposition of chromium having a. density of about 20 Baume' and comprising essentially 100 parts by weight of lchromic acid and about 28 to 56 parts of the mixture obtained by reacting together in aqueous solution the sulphate of a metal Whose dichromate is soluble in water, its oxide, the carbonate of an alkaline earth metal of the group consisting of barium, strontium and calcium, and chromic acid, the sulphate and carbonate being in substantially combining proportions.

5. An aqueous bath for the electrodeposition of chromium having a density of approximately 20 Baum and comprising chromic acid and means for ionizing the bath and obtained by chemical reaction of the product (A), obtained by mixing together the sulphateand the oxide of one of the metals selected from the group consisting of zinc, nickel and aluminum, with chromic anhydride and water, with the product (13) obtained by mixing together the carbonate or" one of the alkaline earth metals selected from the group consisting of barium, strontium and calcium, the selected oxide, .chromic anhydride and water, the sulphate and carbonate being employed in substantially combining proportions, so that the concentrations of sulphate ion in the bath corresponds substan tially only. to the solubility of the alkaline earth metalsulphate, the quantity of chromic anhydride being at least sufiicient to react completely with all of the oxide and carbonate, the bath containing from 2 to v10 parts of the metallic zinc, nickel or aluminum radical for each 100 parts of chromic acid.

6. A composition for use in a bath for the electrodeposition of chromium, which is composed of the mixture obtained by reacting together in aqueous Solution the sulphate of a metal which can form a soluble dichromate, the carbonate of .an alkaline earth metal, and chomic acid, where the S04 radical and the carbonate of the alkaline earth metal chosen are in equivalent amounts, whereby the bath contains a concentration of sulphate ion corresponding substantially only to the solubility of the alkaline earth metal sulphate, the quantity of chromic acid being at least suflicient to react completely with all of the alkaline earth metal carbonate.

7. A composition for use in a bath for the elec trodeposition of chromium, comprising the interacted mixture of (l) a reaction product of RZ-l-R oxide+CrO3 +H2O and (2) a reaction product of XY-l-R oxide+CrOs +H2O in which R is a metal of the group consisting of zinc, nickel and aluminum, Z is the sulphate radical, X is a metal of the group consisting of barium, strontium and calcium, while Y is an anion which is readily decomposed and removes itself from further action in the reaction product, RZ and KY being employed in substantially combining proportions, whereby the concentration of the sulphate ion in the bath corresponds substantially only to the solubility of the sulphate formed in the interacted mixture, the quantity of chromic anhydride being at least suflicient to react completely with all of the oxide and carbonate.

8. An aqueous, inorganic chromium plating bath, containing in solution chromium trioxide and a dichromate of a metal of the group consisting of zinc, nickel and aluminum, the bath being in chemical equilibrium with an undissolved quantity of a sulphate of a metal of the group consisting of barium, strontium and calcium, the sulphate ion content of the bath corresponding substantially only to the solubility of said sulphate, the bath containing about 2 to 10 parts of the metal of the group consisting of zinc, nickel and aluminum for every 100 parts of chromic acid.

9. An aqueous inorganic chromium plating bath containing in solution chromium trioxide and a dichromate of a metal of the group consisting of zinc, nickel and aluminum, and being in chemical equilibrium with an undissolved quantity of a sulphate of a metal of the group consisting of barium, strontium and calcium, the acid content of the bath, other than the chromic acid content, being about .01% to .2% expressed as the percentage of acid radical calculated as equivalent sulphate per one hundred parts of chromic acid, the bath containing from about 2 to 10 parts of the metal of the group consisting of zinc, nickel and aluminum for every 100 parts of chromic acid.

10. A composition for use in a bath for the electrodeposition of chromium, comprising the interacted mixture of (1) a reaction product of RZ+R oxide+CrOs+H2O and (2) a reaction product of XY-l-R oxide-l-CrOH-HzO, in which R is a metal of the grgoup consisting of zinc, nickel and aluminum, Z is a radical group consisting of sulphate, chloride and fluoride, RZ being soluble at least in hot aqueous chromic acid solution, X is a metal which forms an at least difficultly soluble salt with Z, while Y is an anion which is readily decomposed and removes itself from further action in the reaction product, R2 and KY being employed. in substantially combining proportions, whereby the concentration of the anion Z in the bath corresponds substantially only to the solubility of XZ in the mixture, the

quantity of CrOa being at least sufliclent to react completely with all of the oxide and carbonate.

11. An aqueous inorganic chromium plating bath containing in solution chromic trioxide and a dichromate of a metal of the group consisting of zinc, nickel and aluminum, and formed by double decomposition by reaction of a sulphate of such metal with the dichromate of an alkaline earth metal of the group consisting of barium, strontium and calcium, the bath being in chem ical equilibrium with the formed sulphate of the alkaline earth metal, and the sulphate radical .content of the bath corresponding substantially only to the solubility of such sulphate, the bath containing about 2 to 10 parts of the metal of the group consisting of zinc, nickel and aluminum for every parts of chromic acid.

12. A chromic plating bath as set forth in claim 11, wherein the dichromate is zinc dichromate.

13. An aqueous inorganic chromic plating bath containing in solution chromium trioxicle and a dichromate of a metal of the group consisting of zinc, nickel and aluminum, and formed by double decomposition by reaction of a Sulphate of such metal with the dichromate of an alkaline earth metal of the group consisting of barium, strontium and calcium, the bath containing also the formed sulphate of the alkaline earth metal, the acid content of the bath, other than the chromic acid content, having a maximum value of about 0.2% of acid radical calculated as equivalent sulphate per 100 parts of chromic acid, the bath containing about 2 to 10 parts of the metal of the group consisting of zinc, nickel and aluminum for every 100 parts of chromic acid.

14. An aqueous inorganic chromium plating bath containing in solution chromium trioxide and a dichromate of a metal of the group consisting of zinc, nickel and aluminum, and formed by double decomposition by reaction of a sul phate of such metal with the dichromate of an alkaline earth metal of the group consisting of barium, strontium and calcium, the bath being in chemical equilibrium with the formed sulphate of the alkaline earth metal, and the sulphate radical content of the bath corresponding substantially only to the solubility of such sulphate, the quantity of free chromic acid and of the said dichromate in the bath being so related that a dense, adhering deposit of chromium is formed on a cathode upon the passage of an electric current through the bath.

15. The method of electrodepositing chromium which comprises electrolyzin an aqueous solution of chromic oxide containing in solution a chromate or dichromate of a metal selected from the group consisting of zinc, nickel, aluminum, cadmium, trivalent chromium, magnesium, tin, trivalent molybdenum, lithium, sodium and potassium, and a catalytically acting ion whose concentration is determined by the presence of an excess, in the solid phase, of a relatively insoluble sulfate of a third metal of the group consisting of barium, strontium, calcium and silver, and corresponds substantially only to the solubility of said relatively insoluble sulfate.

16. The method according to claim 15, wherein the relatively insoluble salt is barium sulphate.

17. An aqueous bath for the electro-deposition of chromium and containing chromic acid, a soluble dichromate of a metal other than hexavalent chromium and selected from the group consisting of zinc, nickel, aluminum, cadmium, trivalent chromium, magnesium, tin, trivalent molybdenum, lithium, sodium and potassium,

and a relatively insoluble salt of a metal which is more eleotropositive than chromium in the displacement series and is a member of the group consisting of barium, strontium, calcium and silver, said salt being stable in the chromic acid solution and the concentration of the anion of said salt in the bath corresponding substantially only to the solubility of such salt.

18. An aqueous bath for the electro-deposition of chromium and containing chromic acid and two salts of two diiferent metals other than hexavalent chromium, said salts being the relatively soluble dichromate of one metal selected "from the group consisting of zinc, nickel, aluminum, cadmium, trivalent chromium, magnesium, tin, trivalent molybdenum, lithium, sodium and potassium, and a relatively insoluble compound of the other metal said other metal selected from the group consisting of barium, strontium, calcium and silver, and produced by a double decomposition reaction between the salts'of the metals with interchanged anions, said relatively insoluble compound being stable in the chromic acid solution, and the concentration of the anion of said compound in the bath corresponding substantially only to the solubility of such compound.

19. An aqueous bath for the electro-deposition of chromium and containing chromic acid and the dichromate and sulphate of two difierent metals, the first of said metals being selected from the group consisting of zinc, nickel, aluminum, cadmium, trivalent chromium, magnesium, tin, trivalent molybdenum, lithium, sodium and potassium, and the second of said metals being selected from the group consisting of bari- 18 um, strontium, calcium and silver, the dichromate being relatively soluble and the sulphate being relatively insoluble, said dichromate and sulphate comprising the double decomposition reaction product of the salts of said two metals with interchanged anions, having a sulphate ion concentration corresponding substantially only to the solubility of the relatively insoluble sulphate in the bath.

20. An aqueous bath for the electro-deposition of chromium as defined in claim 19, wherein the relatively soluble dichromate is Zinc dichromate. and the relatively insoluble sulphate'is barium sulphate, said bath being free from added sulfuric acid, whereby the generation of hydrogen at the cathode is reduced and the reduction of hexavalent chromium to trivalent chromium minimized, and thereby the accumulation of trivalent chromium prevented, while a high current efficiency is obtained.

FLORICEL A. ROJAS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,050,478 Wickenhiser Aug. 11, 1936 2,114,002 Kramer Q. Apr. 12, 1938 2,172,344 Brown Sept. 12, 1939 OTHER REFERENCES Grant, Hackhs Chemical Dictionary, 1944, page 175. 

1. A COMPOSITION FOR USE IN A BATH FOR THE ELECTRODEPOSITION OF CHROMIUM COMPRISING THE INTERACTED MIXTURE OF (1) THE AQUEOUS REACTION PRODUCT OF 400 GRAMS OF CHROMIC ANHYDRIC,
 41. TO 287.5 GRAMS OF CRYSTALLINE ZINC SULPHATE, 12 TO 81.4 GRAMS OF ZINC OXIDE, AND SUFFICIENT WATER TO DISSOLVE THE INGREDIENTS AND PERMIT THEIR REACTION, AND (2) OF 600 GRAMS OF CHROMIC ANHYDRIDE, 28 TO 197.4 GRAMS OF BARIUM CARBONATE, UP TO ABOUT 81.4 GRAMS OF ZINC OXIDE, AND SUFFICIENT WATER TO SUSPEND THE INGREDIENTS AND PERMIT THEIR REACTION, THE PROPORTIONS OF SULPHATE READICAL AND BARIUM RADICAL BEING SUBSTANTIALLY EQUIVALENT. 